The present invention relates generally to bonding of semiconductor dies in integrated circuit (IC) packages, and more specifically to die collets for picking separated semiconductor dies from a wafer and placing the dies on a die attach pad, lead frame or seating plane to which the die is bonded.
FIG. 1 illustrates a conventional technique for separating a semiconductor die from a silicon wafer. The wafer 10 is mounted on expandable adhesive film 12 secured at its edges by clamps 14. Disposed beneath adhesive film 12 is expansion frame 16 movable in the direction of arrow 18 such that expansion frame 16 pushes upward on film 12 distending it vertically and stretching it horizontally, providing a tense, flat surface on which wafer 10 resides.
Dies 20 are separated from wafer 10 by sawing cuts 22 at the edges of each die. The result is a plurality of dies 20, usually rectangular in shape, mounted on film 12 and separated by the saw kerf at cuts 22.
For placement of the dies on a die attach pad of an integrated circuit (IC) package, each die 20 is lifted from film 12 by a collet 24, which is usually mounted at the end of a pick-and-place device, such as a robot arm or other mechanical actuator. Die collets for placement of semiconductor dies on die attach pads for bonding are known in the art, and are commercially available from, for example, Small Precision Tools of Petaluma, Calif. Automated die attach systems employing die collets like the aforementioned are also well known, being commercially sold by, for example, Advanced Mechanization Incorporated of Horsham, Pa.
As shown in FIG. 2, the collet 24 has an aperture 26 at its distal end and a vacuum hole 28 extending through the length of collet 24 through which a vacuum pressure is exerted for lifting die 20. Aperture 26 has four sloped walls 30 which, at the distal end of aperture 26 are spaced apart a distance such that die 20 can fit within aperture 26.
As shown in FIGS. 3A and 3B, walls 30 of aperture 26 are sloped at an angle such that a gap 34 is disposed between the top of die 20 and end surface 36 of aperture 26. Gap 34 is provided so as to avoid damaging the top surface 38 of die 20. Thus, contact between collet 24 and die 20 is limited to the corners 40 of die 20.
Referring again to FIG. 1, a die 20 is lifted by collet 24 from film 12 by positioning aperture 26 over a die 20, exerting a vacuum pressure in the direction of arrow 32 and simultaneously moving an ejector pin 42 upwardly against the lower surface of film 12. Ejector pin 42 assists collet 24 in removing die 20 from adhesive film 12. Collet 24 is then moved upwardly away from film 12, with die 20 preferably in a position shown in FIG. 3A.
Collet 24 is then positioned over a die attach pad or lead frame where the die is to be bonded. As shown in FIG. 4, the die 20 is placed over pad 44 with a layer of bonding material 46, such as solder, disposed between die 20 and pad 44. Heat is then applied, usually through pad 44, so as to melt solder 46, bonding die 20 to pad 44.
Using this known technique of die placement, several significant problems may arise. One such problem is shown in FIG. 5. As shown by the dotted line, die 20 can become tilted in aperture 26 of collet 24. This can occur for various reasons, such as the upward force of ejector pin 42 when the die is removed from film 12, the force of vacuum pressure through vacuum hole 28, the inertial force as the collet is moved in position over a die attach pad, or the uneven surface of the solder layer 46.
Whatever the reason, the result of the tilting of die 20 is shown in FIG. 6. Die 20 is bonded in a tilted position, with bonding material 46 having a thickness greater on one side than on the other. The tilt of die 20 and the varied thickness of bonding material 46 may lead to device failure during electrical test or during operation in the field because of the uneven thermal gradient across the die.
Another significant problem which arises in known processes is chipping or cracking of the corners of die 20. Such chipping or cracking can take the form shown in FIG. 7, wherein corners 40 contacting walls 30 of aperture 26 have been damaged either when the die 20 is lifted from film 12, or when die 20 is bonded to pad 44. Further, chipping and cracking can occur also at the lower corner 50 of die 20, in the illustration of FIG. 6. This is thought to result from the concentration of stresses at corner 50 when a tilted die in collet 24 is placed on pad 44.
A further problem results from the requirement that aperture 26 be sealed at edges 40 of die 20 in order to provide a proper vacuum seal to retain die 20 in the aperture. Known collets suffer from poor sealing, and have a significant incidence of dropped dies.
Moreover, these problems with tilted and damaged die result in a significant amount of machine down time in the manufacturing process. Currently, when a die is tilted in the aperture 26 of die collet 24, or corners 40 of die 20 are chipped or cracked, either the die is placed on the bonding pad and bonded in the tilted or cracked condition, or, if the machine operator notices the problem, the die attach machine must be stopped to adjust the position of the die so that it will be placed parallel to the die attach pad. A cracked or chipped die must be discarded and replaced.
Accordingly, a method and apparatus for placing a die on a die attach pad or lead frame is desired which would (a) maintain the back surface of the die in a desired plane, (b) reduce or avoid die cracking and/or chipping on or around the corners and edges of the die, (c) achieve a uniform thickness of bonding material between the die and the pad or lead frame, (d) reduce machine downtime attributed to correcting the tilting problems and recovering dropped dies, and (e) reduce the defect rate and test yield loss due to poor attachment of dies.